Part 5

You have all heard about the DNA double helix and genes. Many of you know that mutations occur randomly, that the DNA sequence is read by successive groups of three bases (the codons), that many genes encode enzymes, and that gene expression can be regulated.
These concepts were proposed on the basis of astute genetic experiments, as well as often on biochemical results. The original articles were these concepts appeared are however not frequently part of the normal curriculum of biologists, biochemists and medical students.
This course proposes to read study and discuss a small selection of these classical papers, and to put these landmarks in their historical context. Most of the authors displayed interesting personal histories and many of their contributions go beyond not only the papers we will read but probably all their scientific papers.
Our understanding of the scientific process, of the philosophy underlying the process of scientific discovery, and on the integration of new concepts is not only important for the history of science but also for the mental development of creative science.

从本节课中

Session 5

Before Benzer’s experiments, most genetic mapping required a screening of all the progeny of a cross to calculate the recombination frequencies. The power of the rII system of phage T4 resides in the facts that many independent mutants can be identified by scoring plaque morphology on the permissive host strain, and that only wild type recombinants will grow on the restrictive host strain. This, together with the very high recombination of T4 DNA, allows the detection of recombination between mutations affecting adjacent nucleotides. The cis-trans complementation test showed that the rII locus consists of two genes. Using more than 100 deletion mutants, which do not revert to wild type, Benzer first demonstrated that the topology of DNA is linear. Using these deletions, he was able to carve the rIIA and rIIB genes in 47 segments. A very simple and rapid recombination test allowed him to map thousands of point mutants, both independent and mutagen-induced, in individual segments. This map was perfectly congruent with maps laboriously constructed by classical recombination tests. The topography of the map was strikingly non-random, with sites that are 100 times more likely to mutate than others and are called hot spots. The spectrum of sites detected after mutagenesis was strikingly different. It is now known that most of the spontaneous rII mutants are frame-shift, i.e. addition or deletion of one or a few base pairs that disrupt the translation of the mRNA into protein. In contrast, most mutagens used in this work induce base substitutions that do not usually arrest translation. Now that the sequence of the rII locus is known, the saturation of the map is such that there are about one mutation every 8 nucleotides.